The present invention relates to compounds that are useful for surface modification and corrosion inhibition of metallic particles, such as metallic flake pigments and in particular aluminum flake pigment. In particular, the invention concerns decorative coatings formed from aqueous compositions containing metallic flake pigments that are surface modified with such compounds.
The use of metallic flake pigments, such as aluminum flake pigments in decorative coatings to give the coating a metallic effect is widespread. The metallic effect is particularly popular with customers in the automotive market, and metallic colors form a large share of automotive paints.
Automotive coatings may utilize a single, uniformly pigmented layer. Alternatively, they may have two distinct layers, a lower, highly pigmented layer and an upper layer with little or no pigmentation. The two-layer coating is known in the industry as basecoat/clearcoat or color plus clear coat. Basecoat/clearcoat coatings impart a high level of gloss and depth of color that result in a particularly appealing look. Metallic flake pigments may be incorporated to give the coating a metallic effect.
Waterborne automotive paints are gaining widespread usage in the automotive industry due to concerns over organic solvent emissions during application of paints. The new waterborne paints have the disadvantage of using a medium which is corrosive to metallic flake pigments. For example the pH of the waterborne acrylic coating compositions typically ranges from 8.0-9.0, and the polyurethane coating compositions have a pH typically ranging from 7.5 to 8.0. In a basic pH environment, the aluminum pigment is oxidized. The oxidation is a form of corrosion which destroys the metallic pigmentation properties of the mirror-like particles. When a paint with oxidized metallic flake pigments is coated onto a substrate, the coating shows discoloration and diminished metallic effect.
Furthermore, oxidation of the metallic surfaces by water results in the evolution of hydrogen gas. The amount of hydrogen gas evolved is indicative of the amount of oxidation (i.e. corrosion) of the metallic pigment. The hydrogen gas may accumulate in the paint.
Deterioration of aluminum pigment may accelerate over time due to continuous contact with the basic pH environment of the coating composition. Coating compositions containing metal flake pigment are often stored for 6 months or more before application, which can result in significant corrosion of the pigment. If this corrosion remains unchecked the coating composition may be unusable.
Considerable work has been done in the industry to try to protect the metal surfaces from water in the waterborne coating compositions. The pigments may be made corrosion resistant by treatment with transition metal salts, rare earth metal salts or mixtures thereof, as disclosed in U.S. patent applications having Ser. No. 07/982,355 and 07/982,352.
In addition, the pigments may be treated with silane or phosphorus substituted polymeric compounds, as taught by U.S. Pat. No. 5,156,677 to Carpenter et al.
It has been discovered that metallic flake pigments that are surface modified with the compounds of the present invention are particularly resistant to oxidation in waterborne paints with minimal discoloration or diminution of the metallic effect in the coating. It has also been discovered that metallic flake pigments surface modified according to the present invention form superior dispersions in water in comparison to metallic flake pigments using previously known surface treatments. This superior dispersion in the waterborne paint composition results in a coating with an enhanced metallic effect.